1. Field of the Invention
The present invention relates to a cylindrical roller bearing comprising a cage made of a synthetic resin, and particularly to a cylindrical roller bearing used to support a rotating member that rotates at high speed, such as the main spindle in a machine tool.
2. Description of the Related Art
It is well known that in machine tools such as machining centers, CNC lathes, and milling machines, the main spindle is usually supported by a bearing in a freely rotatable manner relative to the housing. The orientation of this main spindle is divided into two broad categories, vertical (where the axis lies along the vertical) and horizontal (where the axis lies along the horizontal), depending on the type of machine. Furthermore, although dependent on the actual conditions of use, oil lubrication such as grease lubrication and air oil lubrication and the like are typically used as the lubricating method for the bearing which supports the spindle, and combined angular contact ball bearings or cylindrical roller bearings or the like are the most widely used types of bearing.
In this case, cylindrical roller bearings used to support the spindle of the machine tool typically comprise a cage, which holds a plurality of cylindrical rollers at predetermined intervals between an inner ring and an outer ring. Conventionally, cages machined from high strength cast brass have been the most widely used, but from the perspective of factors such as lubricant degradation due to abrasive powder produced by the cage during operation, and weight reduction, a move has recently begun toward cages made of a synthetic resin.
With this transition in technology, one known example of a synthetic resin cage that is currently in use within cylindrical roller bearings is the so-called comb type cage, as disclosed in Japanese Patent Laid-Open Publication No. Hei 11-166544 (referred to as patent reference 1 below) and International Patent Publication No. WO 03/029670 (referred to as patent reference 2 below). This comb type cage comprises a ring section, a plurality of pillars extending in one axial direction from the inside surface of the ring section, and a plurality of pockets formed between the surfaces on the circumferential direction side of pillars positioned adjacently in the circumferential direction, the pockets holding the cylindrical rollers in a freely rotatable manner. However, in a synthetic resin cage of this construction, because the tips of the pillars are free edges, the pillars undergo a comparatively large amount of outward elastic deformation due to the effect of centrifugal force generated during rotation, particularly at high rotation speeds, and the inner circumferential sections of these tips on the circumferential direction side faces can come into strong contact (abnormal contact) with the rolling contact surfaces of the cylindrical rollers, causing a deficiency of oil film between the contacting parts, which can generate abnormal abrasion or a rise in the bearing temperature.
In response to these problems, in the patent references 1 and 2, the circumferential direction side face 14b11 of a pillar 14b that extends in the radial direction from an annular section 14a of a cage 14 is divided into an outside diameter region and an inside diameter region, with the rolling element PCD that passes through the center O of a pocket 14c acting as the boundary, as shown in FIG. 23. The outside diameter region is formed as a circular arc surface (cylindrical surface) 14b11 that follows the rolling contact surface of the cylindrical roller 13, and the inside diameter region is formed as a straight surface 14b12 along the entire radial length, in parallel with a radial line r1 that passes through the pocket center O, thereby preventing contact pressure from occurring in the radial direction between the inside diameter region 14b12 of the circumferential direction side face 14b1 of the pillar 14 and the rolling contact surface of the cylindrical roller 13 when the centrifugal force generated during rotation causes outward elastic deformation of the pillar 14b. 
Examples of essential properties required of a main spindle in a machine tool are the ability to undergo high speed rotation {normally where the dmn value (=pitch circle diameter of the rolling element (mm)×number of revolutions (rpm)) is at least 1,000,000} and that non repeatable run-out (NRRO) is minimal, and these characteristics are primarily determined by the axial bearing function of the bearing that supports the spindle. However, for the reasons described below, with the cylindrical roller bearings disclosed in the patent references 1 and 2, it is difficult to obtain the level of non repeatable run-out (NRRO) required for the main spindle of a machine tool.
In other words, in the cage 14 of the cylindrical roller bearings disclosed in the patent references 1 and 2, as described above, the region of the circumferential direction side face 14b1 of the pillar 14b from the rolling element PCD toward the inside diameter is formed as a straight surface 14b12 along the entire radial length, meaning that contact pressure in the radial direction does not occur between the inside diameter region 14b12 of the circumferential direction side face 14b1 and the rolling contact surface of the cylindrical roller 13 when the centrifugal force produced during rotation causes outward elastic deformation of the pillar 14b. However, although this construction is effective in terms of preventing abnormal contact between the circumferential direction side face 14b1 of the pillar 14b and the rolling contact surface of the cylindrical roller 13, forming the inside diameter region of the circumferential direction side face 14b1 of the pillar 14b as the straight surface 14b12 described above has the result of promoting further outward elastic deformation of the pillar 14b. In other words, by forming the inside diameter region of the circumferential direction side face 14b1 of the pillar 14b as the straight surface 14b12 described above, there are fewer locations where outward elastic deformation is restricted than in the normal pocket shape (in which the entire region of the circumferential direction side face of the pillar is formed as a circular arc surface that follows the rolling contact surface of the cylindrical roller). Furthermore, the circumferential direction wall thickness of the inside diameter region of the pillar 14b is also reduced, which lowers the rigidity of the pillar 14b, resulting in the promotion of outward elastic deformation by the pillar 14b. 
FIG. 24 shows a schematic representation of a state where the pillars 14b of the cage 14 of the cylindrical roller bearing disclosed in the patent references 1 and 2 have undergone outward elastic deformation due to the centrifugal force generated during high speed rotation (the solid lines), and a state prior to such deformation (the dashed lines). As shown in the figure, with the cage 14 of the cylindrical roller bearing disclosed in the patent references 1 and 2, when the pillar 14b undergoes outward elastic deformation, the pocket gap g between the circumferential direction side face 14b1 of the pillar 14b and the rolling contact surface of the cylindrical roller 13 increases from the initial gap (the gap prior to deformation). Moreover, as a result of the promotion of further outward elastic deformation of the pillar 14b described above, the pocket gap g is further increased. An increase in this pocket gap g causes a reduction in the equalization capacity of the cylindrical roller and run-out of the center of revolution of the cylindrical roller, and non repeatable run-out occurs, leading to unstable shaking of the inner ring. In particular, with roller guiding cages, increasing the degree of freedom of the cage in the radial direction produces locations where the pocket gap g increases and locations where the pocket gap g shrinks, and the positions of these locations are not constant, which increases the degree of non repeatable run-out. This non repeatable run-out (NRRO) increases in proportion to the number of revolutions, and causes a variety of problems, including a deterioration in the machining accuracy of tools mounted to the main spindle of the machine tool.
FIG. 25 is a representative longitudinal cross-sectional view (FIG. 1 in the patent reference 1) showing the relative dimensional relationship between the annular section 14a and the pillar 14b of the cage 14, and the cylindrical roller 13 in the cylindrical roller bearing described in the patent references 1 and 2. As shown in FIG. 25, the length (thickness) Ta in the axial direction of the annular section 14a of the cage 14 is set to approximately 25% of the length Td in the axial direction of the cylindrical roller 13. In other words, as can be understood from the other diagrams in the patent references 1 and 2, in conventional synthetic resin cages, the dimensional relationship above was generally set to a value of approximately 25%. The reasons for such settings are because the relationship between the length Te in the axial direction of the inner ring 11 and the outer ring 12 and the length Td in the axial direction of the cylindrical roller 13, taking into consideration the function of the cylindrical roller bearing, is substantially the same value each time. If the design is based on this value and the characteristics of the resin material of the cage 14, then the length Ta in the axial direction of the annular section 14a of the cage 14 must naturally be set to approximately 25% of the length Td in the axial direction of the cylindrical roller 13.
However, while a cylindrical roller bearing according to this conventional design demonstrates adequate capabilities at low rotation speeds, at high rotation speeds there is a danger of elastic deformation of the pillars 14b of the cage 14 causing the bearing temperature to rise excessively. Furthermore, even when the method disclosed above in the patent references 1 and 2 is used, and a straight surface 14b12 is formed on the pillar 14b of the cage 14 with the conventional design described above, at high rotating speeds in excess of 13,000 rpm or thereabouts for example (or when the dmn value exceeds 1,650,000 or thereabouts), the bearing temperature may still rise excessively. It is fair to say that the cause of this phenomenon is the effect of centrifugal force causing the pillars 14b of the cage 14 to undergo a large amount of outward elastic deformation.
Furthermore, the inventors of the present invention have discovered, as a result of considering measures for preventing contact between the pillars 14b of the cage 14 and the cylindrical roller 13, that the phenomenon wherein the bearing temperature rises excessively at high rotational speeds due to elastic deformation of the pillars 14b depends largely on the length Ta in the axial direction of the annular section 14a of the cage 14. Accordingly, if the dimensional relationship between the length Ta in the axial direction of the annular section 14a of the cage 14 and the length Td in the axial direction of the cylindrical roller 13 is set as in conventional designs, there is a concern that this may greatly impede the ability to adequately reduce the elastic deformation of the pillars 14b caused by the effect of an extremely large amount of centrifugal force.
In addition, with such a cage 14 of a cylindrical roller bearing, if the pillars 14b are held such that they are in contact with the cylindrical roller 13, then the relationship between the two causes abrasion and the like due to contact resistance and sliding resistance, and to counter this, a film of a lubricant such as grease or air-oil is formed between the two. However, when a simple straight surface 14b12 is formed on the circumferential direction side face 14b1 of the pillar 14b, as in the cage 14 of the cylindrical roller bearing disclosed in the patent references 1 and 2, contact during rotation, and particularly contact or sliding during rotation over long periods of time, increases the probability of a deficiency of oil film occurring between the pillar 14b and the cylindrical roller 13, which inevitably leads to a deterioration in the lubrication properties. Despite this, the above patent references give no consideration to countermeasures for reliably preventing deterioration of the lubrication properties, and there is not even any indication or suggestion that the inventors were aware of such a problem. At present, an appropriate solution to this problem is still required.